<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script>
        window.addEventListener('load', function () {
            var btn = document.querySelector('button');
            btn.addEventListener('click', function () {
                alert('点击我')
            })
        })
        window.addEventListener('load', function () {
           alert(22);
        })
        document.addEventListener('DOMContentLoaded',function(){
            alert(33);
        })
    </script>
</head>

<body>
    <button>点击</button>
</body>

</html>